U.S. President Barack Obama and Republican presidential nominee Mitt Romney shake hands at the end of the third and final presidential debate October 22, 2012 at Lynn University in Boca Raton, Florida.(MANDEL NGAN/AFP/Getty Images)

WASHINGTON -- The political fortunes of Barack Obama and Mitt Romney could be determined today in little-known counties with names like Loudoun, Lucas and Larimer.

After 17 months and more than $2 billion spent crisscrossing the nation in search of votes, the two presidential candidates, their staffs and an army of journalists and political junkies will huddle tonight over county maps and precinct reports to gauge the voters' verdict.

It won't be New York, Chicago or Los Angeles that decides the election. Instead, swing counties such as Virginia's Loudoun, Ohio's Lucas and Colorado's Larimer will play outsized roles in picking the next president.

Today, turnout among Democrats' and Republicans' base voters will begin to tell the tale of the 2012 election. Turnout was above 65% in 10 swing states in 2008 and 61.6% overall, slightly higher than 2004.

Then the TV networks and other news organizations will begin releasing the results of "exit polls" taken outside polling places, which will shed light on the choices made by men and women, whites and minorities, young and old, urban and rural. More so than in the past, that information is likely to spill out on Twitter and other social media outlets.

Once the polls close from east to west, the results from early voting - representing as much as two-thirds of the vote in some states - will become known. That will be followed by the gradual tabulation of Election Day votes, starting at 7 p.m. ET and continuing deep into the night.

All the clues will be important, because news organizations may be cautious in projecting state winners from a combination of exit polls, vote counts from key precincts, the number of votes outstanding and historical voting data. That's partially a result of the debacle of 2000, when TV networks prematurely proclaimed George W. Bush president long before that result became clear. In 2004, exit polls greatly overstated Democrat John Kerry's strength in his failed effort to unseat Bush.

Here's a guide to watching the returns tonight:

7 P.M. IN VIRGINIA

Exurbs rule

There was a reason the president kept returning to Prince William County, a Washington, D.C., exurb, in the campaign's waning days. Together with neighboring Loudoun County, both just beyond reliably Democratic Fairfax, it offers the keys to the Old Dominion.

"They're exurbs, and they're diverse. The big breakthrough for Obama (in 2008) came when he won those counties," says Larry Sabato, who directs the University of Virginia's Center for Politics.

Other keys: Henrico and Chesterfield counties, around Richmond, and military-dependent Virginia Beach, the state's most populous city. Romney must run strong there; in 2008 Obama held his own, even against war hero John McCain.

As for demographics, Sabato will look primarily at gender and race. Obama almost surely will win among women and Romney among men, but whoever enjoys the greater gender gap will win. And the president must get close to 40% of the white vote to hold on, Sabato says.

Virginia most closely mirrored the national election four years ago, giving Obama 52.6% of the vote (he won 52.9% nationally). If one candidate is declared the winner in Virginia fairly early in the evening, Sabato says, "that candidate is very likely to win the election, because that means either Romney or Obama is running well ahead of expectations."

7:30 P.M. IN OHIO

Auto country

There's something different to watch here this year beyond the usual Democratic, Republican and bellwether counties: auto country.

Traditionally Democratic, the counties around Youngstown and Toledo could be even more so this year, and that would be welcome news for Obama. If the president polls 60% or more there, he could win the state, University of Akron political science professor David Cohen says.

Perhaps most telling will be Lucas County, which Cohen calls "ground zero" of the latest Ohio flap: Romney's last-minute ad implying that Chrysler, which runs a Jeep plant in Toledo, will ship jobs to China.

The state that most often determines the presidency also has several key swing counties: Look to Lake County east of Cleveland, where Obama visited Saturday, as well as Canton-based Stark County, both of which usually pick the winners.

Obama won the male vote here in 2008, exit polls showed - something he's unlikely to repeat, so he needs a healthy majority of women, Cohen says. He also needs to crush Romney by more than 2-to-1 in Cuyahoga County (Cleveland), while a strong showing by Romney in Hamilton County (Cincinnati) could indicate the state is going Republican.

NORTH CAROLINA

A turnout battle

The African-American vote in North Carolina gave Obama one of his two most unlikely victories in 2008 (Indiana was the other). The question this year: Will blacks turn out in sufficient numbers to do it again?

Most polls suggest not, giving Romney one of his only clear swing-state advantages. Still, gains among Latino voters, plus the Obama campaign's vaunted get-out-the-vote effort, could make things close.

"Electing a black president made history. Re-electing a black president isn't the same thing," says David Rohde, a political science professor at Duke University.

He will look to the state's heavily minority counties such as Durham, where Duke is based, for signs that turnout is as strong or weaker than it was in 2008, when turnout increased over previous years by more than any other state.

8 P.M. IN FLORIDA

Late night ahead?

If the election is close in the Sunshine State, as it usually is, the most important votes won't show up in either candidate's column as the results are tabulated.

They are the provisional ballots, those that election officials must certify later, and their number is expected to grow this year because of rules relating to address changes. Historically, Democrats have been twice as likely to cast provisional ballots.

"All eyes are going to be on them," University of Florida political science professor Daniel Smith says of the provisional votes. "And all the lawyers' eyes are going to be on them."

About half the state's voters have cast early ballots, and those should be tabulated fairly quickly after the polls close. Watch Hillsborough County in the Tampa Bay area for a clue to the election, Smith says - its ethnic and racial diversity tipped 53% of its voters for Obama in 2008.

NEW HAMPSHIRE

Manchester is key

The biggest city in the state closes its polls at 7 p.m., ahead of some others, and usually tallies results quickly. So shortly after all state polls close at 8 p.m., its results should be known.

Andy Smith, director of the University of New Hampshire Survey Center, says Obama needs at least 53% in Manchester to capture the state. Anything less could be overwhelmed by the small, rural counties that will report much later and tilt Romney's way. "Just watch Manchester. That's going to give you a really good idea," Smith says. "It sets the tone for the entire state."

PENNSYLVANIA

East meets west

Romney's late effort to steal the state from Obama's win column will hinge on turnout in two areas: east of the Susquehanna River in Philadelphia and its suburbs, where Obama should get a majority of at least 500,000, and northern and southwestern Pennsylvania, where Romney needs to pile up white, blue-collar votes.

But Pennsylvania also has eight swing counties in the east that could determine the election: Bucks, Montgomery, Chester and Delaware counties, which surround Philadelphia, will be watched closely, as will Lehigh, Northampton, Monroe and Berks, which include cities such as Allentown, Bethlehem and Reading.

Those counties have the largest number of independents, says Terry Madonna, director of the Center for Politics and Public Affairs at Franklin & Marshall College in Lancaster, Pa. The candidate who captures most of those counties could win the state, he says, "because you're winning the voters in the middle."

MICHIGAN

Two key swing counties

For years, political prognosticators looked to Macomb County north of Detroit, home of the "Reagan Democrats," as the key to Michigan politics. Now it's got competition from neighboring Oakland County.

Oakland gave 57% of its votes to Obama in 2008 and Macomb 53%. If Obama can just break even or come close this year in Macomb while holding Oakland, "then obviously Republicans are in bad shape," says political analyst Ed Sarpolus. "There's no way that Romney can win losing Oakland County." Another place to watch is Kent County, home of Grand Rapids, where the strong Republican base has been eroding.

This is another state where Obama needs to win big among blacks and union workers to compensate for a likely loss among whites. If Romney can get 55% among whites, Sarpolus says, he could have a good night.

9 P.M. IN COLORADO

Watch the early vote

Some 70% of Colorado voters' ballots were cast before Election Day and will be counted quickly after the polls close. Ironically, however, this is one state where the result may hinge on the last votes to be counted - late postmarks, slow precincts, provisional ballots. It's that close.

Both parties have their base counties where they should perform well; who does better depends on turnout, which topped 70% in 2008. Then there are the swing counties: Jefferson and Arapahoe around Denver, and Larimer on the Wyoming border, which includes Fort Collins.

Obama won all three counties in 2008 with 54%-55% of the vote, but this is a closer election. "You've got to watch what happens in those swing counties," says Floyd Ciruli, a Colorado pollster and political analyst.

The president will win women, Latinos and young people, Ciruli says, but the margins will be telling.

WISCONSIN

North by northwest

Milwaukee, Madison and Green Bay won't decide this election. Instead, look to the north and northwest. That area has been the bellwether of late. It went for Obama in 2008 but swung back to elect Republican Scott Walker governor in 2010 and help him survive a recall this past June.

"When Democrats win statewide, they normally win a majority of the counties in that area," says Charles Franklin, director of the Marquette University Law School Poll. "If that area looks mostly blue, that's great news for Obama. If it looks mostly red, that's a real warning, and probably good news for Romney."

Another wild card is Paul Ryan, the GOP candidate for vice president. His base in Rock County is usually Democratic turf - Obama won it with 64% last time - but Ryan's name is on the ballot twice as he seeks to retain his seat in Congress should the Romney-Ryan ticket lose.

MINNESOTA

Ballot issues could help GOP

How this blue state nudged its way into the November discussion is a mystery to many. The answer might come from elsewhere on the ballot.

If Obama has trouble, it could indicate conservative enthusiasm for traditional marriage and tougher voter ID rules, questions that are being put to voters. "If they're passing, that's an early indication that it's a good night for Republicans," says Steven Schier, political science professor at Carleton College in Northfield, Minn.

Obama needs 60% or more in the Minneapolis-St. Paul area to compensate for Romney's advantage in the suburbs and exurbs, Schier says. "That's where Republicans really have to stop the Democrats," he says. Turnout in Minnesota in 2008 was 78%, the highest in the nation.

10 P.M. IN IOWA

Shades of 2008?

The early vote will be tabulated quickly after polls close, giving the first sign of where things stand. By all indications, it should show a Democratic lead of as much as 10 points.

Heavy turnout in Iowa's college towns, such as Ames, Cedar Falls and Iowa City, could be driving that lead, but whether that turnout matches 2008 remains to be seen. "There's some concern that younger voters are not going to turn out for Obama the way they did in 2008," says Steffen Schmidt, a political science professor at Iowa State University.

Several other races in Iowa could provide hints for the presidential race: Watch the retention battle over Judge David Wiggins, a gay marriage proponent; Iowa Senate Democratic leader Mike Gronstal's difficult re-election, and Christie Vilsack's uphill challenge to Republican Rep. Steve King for indications of Democratic strength or weakness, Schmidt says.

NEVADA

Las Vegas area dominates

One county dominates the political landscape here: Clark, home to Las Vegas and 70% of the state's voters. Obama led there by 120,000 votes in 2008, representing his entire margin statewide, and anything close to that would signal victory again.

Then comes Washoe County, home to Reno, and one of only two other counties Obama won four years ago. Romney needs to win there, says David Damore, associate professor of political science at the University of Nevada-Las Vegas.

Much will be known right after the polls close, because early voters make up roughly two-thirds of the state's voters, and those results will come in fast. "If Romney's not up in the early voting in Washoe, it's really tough for a Republican," Damore says.

If the election is as close as polls suggest, it might come down to absentee ballots that are still in the mail and tens of thousands of "provisional" ballots that won't be counted until after Election Day. In Ohio, they can't even be reviewed for 10 days.

"Even without hanging chads, we could be into December before we know the results," Rohde says.